Single focus lens

ABSTRACT

A single focus lens comprises: a first lens of positive power having a convex-shaped surface on an object side; a second lens of a negative meniscus lens having, on the object side, a concave-shaped surface on its paraxial axis; and a third lens of an aspheric lens having, on the object side, a convex-shaped surface on its paraxial axis, in this order from the object side, wherein the single focus lens satisfies the predetermined conditions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a single focus lens favorably used fora small-sized apparatus having an image pick-up feature, for example, amobile telephone having a camera feature, a PDA (Personal DigitalAssistant), and a digital still camera, etc.

2. Description of the Related Art

An image pick-up device such as a CCD (Charge Coupled Device) and a CMOS(Complementary Metal Oxide Semiconductor) is used in an image pick-upapparatus such as a digital still camera. In recent years, such an imagepick-up device has been further remarkably downsized. Therefore, it hasbeen demanded that a pick-up apparatus and a lens built in the pick-upapparatus are downsized and reduced in weight. Also, in recent years, animage pick-up device having a large number of pixels has been developedin order to achieve high image quality. In line therewith, performanceby which high resolution and high contrast can be brought about isrequired for the lens system.

For example, a pick-up lens described in the following JP-A-10-48516 isavailable as an image pick-up lens used for such an image pick-upapparatus. JP-A-10-48516 describes an image pick-up lens composed ofthree lenses which are the first, the second and the third lenses inorder from the object side. In the image pick-up lens, the power of thefirst lens is low, and an aperture stop is disposed between the secondlens and the third lens.

In regard to recent image pick-up devices, downsizing and highconcentration of pixels have been advanced as described above. In linetherewith, high resolution performance and compacting in structure hasbeen demanded for an image pick-up lens. The image pick-up lensdescribed in JP-A-10-48516 described above has achieved performance andcompactness to some degree with a fewer number of a lenses of three-lensconstruction. However, it is desired that a lens system of furthercompactness and higher performance is developed.

SUMMARY OF THE INVENTION

The present invention has been made in view of such problems, and anobject thereof is to provide a single focus lens which can achieve acompact lens system having high performance while having a fewer numberof lenses.

A single focus lens according to the present invention includes:

-   -   a first lens of positive power having a convex-shaped surface on        an object side;    -   a second lens of a negative meniscus lens having, on the object        side, a concave-shaped surface on its paraxial axis; and    -   a third lens of an aspheric lens having, on the object side, a        convex-shaped surface on its paraxial axis,    -   in this order from the object side,    -   wherein the single focus lens satisfies the following        conditions:        1.5>f1/f>0.6  (1)        24.0<vdA<32.0  (2)        55.5<vdB<58.5  (3)        1.55<NdA  (4)        1.47<NdB<1.55  (5)        1.8>L/D  (6)        0.30<RA/f<0.40  (7)        where    -   f: Entire focal distance,    -   f1: Focal distance of the first lens,    -   vdA: Abbe number of the second lens,    -   vdB: Abbe number of the first lens and the third lens,    -   NdA: Refractive index at d-line of the second lens,    -   NdB: Refractive index at d-line of the first lens and the third        lens,    -   RA: Paraxial curvature radius of a surface at the object side of        the first lens,    -   L: Distance on the optical axis from the surface at the object        side of the first lens to an imaging position, and    -   D: Maximum image height.

A single focus lens according to the present invention may furtherinclude an aperture stop disposed at the object side from the surface atan image side of the first lens on the optical axis.

With the single focus lens according to the present invention, bysatisfying the conditional expressions (2), (3), (4), and (5) andadequately preparing vitreous materials of the respective lenses whilesatisfying the conditional expressions (1), (6) and (7) and securing thecompactness, a compact lens system of high performance can be achievedwhile being composed of only three lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the first configurational example of a single focus lensaccording to one embodiment of the present invention, which is a lenssectional view corresponding to Example 1;

FIG. 2 shows the second configurational example of a single focus lensaccording to one embodiment of the present invention, which is a lenssectional view corresponding to Example 2;

FIG. 3 is a view depicting basic lens data of a single focus lensaccording to Example 1;

FIG. 4 is a view depicting data of an aspheric surface of a single focuslens according to Example 1;

FIG. 5 is a view depicting basic lens data of a single focus lensaccording to Example 2;

FIG. 6 is a view depicting data of an aspheric surface of a single focuslens according to Example 2;

FIG. 7 is a view collectively depicting the values of the conditionalexpressions with respect to the respective examples;

FIG. 8A is an aberration view showing spherical aberration of a singlefocus lens according to Example 1;

FIG. 8B is an aberration view showing astigmatism of a single focus lensaccording to Example 1;

FIG. 8C is an aberration view showing distortion of a single focus lensaccording to Example 1;

FIG. 9A is an aberration view showing spherical aberration of a singlefocus lens according to Example 1;

FIG. 9B is an aberration view showing astigmatism of a single focus lensaccording to Example 1; and

FIG. 9C is an aberration view showing distortion of a single focus lensaccording to Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a detailed description is given of an embodiment of thepresent invention with reference to the drawings.

FIG. 1 is a first configurational example of a single focus lensaccording to one embodiment of the present invention. Theconfigurational example corresponds to a lens configuration of the firstnumerical mode (FIG. 3 and FIG. 4) described later. In addition, FIG. 2is a second configurational example. The configurational example of FIG.2 corresponds to a lens configuration of the second numerical mode (FIG.5 and FIG. 6) described later. In FIG. 1 and FIG. 2, reference symbol Ridenotes the curvature radius of the (i)th surface (i=1 through 8) withthe symbol affixed so that the side of a component at the extreme objectside is made into the first side and the symbol is gradually increasedtoward the image side (that is, the imaging side). Reference symbol Didenotes a between-surface distance on the optical axis Z1 between the(i)th side and the (i+1)th side. Also, since the basic construction iscommon in the respective configurational examples, hereinafter, adescription is given of the construction of a single focus lens depictedin FIG. 1 as the basis.

The single focus lens is mounted in a small-sized apparatus having animage pick-up feature, for example, a PDA (Personal Digital Assistant),a video camera, and a digital camera, etc., and is favorably used. Thesingle focus lens is provided with the first lens G1, the second lens G2and the third lens G3 in order from the object side along the opticalaxis Z1. The aperture stop St is disposed not at the image-side surfaceof the first lens G1 but at the object side thereof on the optical axisZ1. It is preferable that the aperture stop St is disposed between theobject side surface of the first lens G1 and the image-side surfacethereof on the optical axis Z1.

An image pick-up device (not shown) such as a CCD, etc., is disposed onthe imaging plane (pick-up plane) of the single focus lens. Varioustypes of optical components GC maybe disposed between the third lens G3and the pick-up plane in accordance with the configuration at the cameraside in which the lens is mounted. For example, cover glass forprotecting the pick-up plane and flat type optical components such asvarious types of optical filters may be disposed therebetween.

It is preferable that the first lens G1, the second lens G2, and thethird lens G3 are all aspheric lenses made of a plastic material. Thefirst lens G1 has positive power, the object side surface of which isconvex-shaped. The second lens G2 is a negative meniscus lens, theobject side surface of which is concave-shaped on the paraxial axis. Thethird lens G3 is made into an aspheric lens, the side at the object sideof which is convex-shaped on the paraxial axis.

It is preferable that the surface at the object side of the third lensG3 is shaped so that positive power thereof is gradually weakened towardthe periphery. It is preferable that the surface at the image side ofthe third lens G3 is concave-shaped to the image side on the paraxialaxis, negative power thereof is weakened toward the periphery, and poweris turned into positive power at the periphery.

The single focus lens satisfies the following conditional expressions,where f is the entire focal distance, f1 is the focal distance of thefirst lens G1, vdA is the Abbe number of the second lens G2, vdB is theAbbe number of the first lens G1 and the third lens G3, NdA is therefractive index at d-line of the second lens G2, NdB is the refractiveindex at d-line of the first lens G1 and the third lens G3, RA is theparaxial curvature radius of the surface at the object side of the firstlens G1, L is the distance on the optical axis from the surface at theobject side of the first lens to an imaging position, and D is themaximum image height. L is made into a value in which the thickness ofan optical component such as a cover glass is converted into air.1.5>f1/f>0.6  (1)24.0<vdA<32.0  (2)55.5<vdB<58.5  (3)1.55<NdA  (4)1.47<NdB<1.55  (5)1.8>L/D  (6)0.30<RA/f<0.40  (7)

Next, a description is given of actions and effects of the single focuslens constructed as described above. By satisfying the conditionalexpressions (2), (3), (4), and (5) and optimizing vitreous materials ofthe respective lenses in refractive index and dispersion characteristicswhile satisfying the conditional expressions (1), (6) and (7) andsecuring the compactness, a compact lens system of high performance canbe achieved while being composed of only three lenses as describedabove.

The conditional expression (1) pertains to the focal distance of thefirst lens G1. It is not preferable that the entire length becomesexcessively long if exceeding the numerical range, and the pupil becomesexcessively short if less than the numerical range. The conditionalexpression (6) expresses the ratio of the entire optical length on theoptical axis Z1 to the maximum image height D. If exceeding thenumerical value, it is not possible that the entire lens system is madesufficiently compact. The conditional expression (7) pertains to thecurvature radius of the front surface of the first lens G1. It is notpreferable that it becomes difficult to shorten the entire length of thelens if exceeding the numerical range, and that it becomes difficult tocorrect the spherical aberration and image surface curvature. In thesingle focus lens, the entire length is attempted to be shortened bymaking small the curvature radius of the front surface of the first lensG1 to make power comparatively large.

The conditional expressions (2) and (4) pertain to the characteristicsof a vitreous material of the second lens G2. If a vitreous materialhaving a small Abbe number and a large refractive index is used for thesecond lens G2 having negative power so that these conditionalexpressions are satisfied, it is possible to favorably correct the coloraberration and image surface curvature. The conditional expressions (3)and (5) pertain to the characteristics of vitreous materials of thefirst lens G1 and the third lens G3. If the Abbe number and therefractive index are within the numerical range, necessary andsufficient performance can be obtained in regard to both the coloraberration and the image surface curvature in a combination with thesecond lens G2, and, as vitreous materials of the first lens G1 and thethird lens G3, it is possible to choose a material, which isinexpensive, easily available and optically well-stabilized (lowdistortion), such as acrylic resin or a cycloolefin polymer.

In addition, in the single focus lens, since the aperture stop St isdisposed not at the image side of the first lens G1 but at the objectside thereof on the optical axis Z1, further preferably between theobject side surface of the fist lens G1 and the image-side surfacethereof on the optical axis Z1, it is possible to shorten the entirelength including the aperture stop St. Further, in the single focuslens, since an aspheric surface is adequately employed for therespective lenses, a great aberration correction effect can be broughtabout. In particular, by adequately setting the aspheric surface shapeof the third lens G3 with respect to the paraxial shape and peripheralshape, a greater effect can be brought about in connection to not onlycorrection of the image surface curvature but also aberrationcorrection.

As described above, according to the single focus lens according to theembodiment, since the predetermined conditional expressions aresatisfied, and the vitreous materials of the respective lenses and powerdistribution are optimized, it is possible to achieve a compact lenssystem having high performance while being provided with only threelenses.

EXAMPLES

Next, a description is given of detailed numerical examples of a singlefocus lens according to the embodiment. Hereinafter, two numericalexamples (Example 1 and Example 2) will be described at a time.

FIG. 3 and FIG. 4 show detailed lens data (Example 1) corresponding tothe configuration of the single focus lens depicted in FIG. 1. Inparticular, FIG. 3 shows the basic lens data thereof, and FIG. 4 showsdata pertaining to an aspheric surface. In addition, FIG. 5 and FIG. 6show the detailed lens data (Example 2) corresponding to theconfiguration of the single focus lens depicted in FIG. 2. Inparticular, FIG. 5 shows the basic lens data, and FIG. 6 shows datapertaining to an aspheric surface.

With the single focus lenses according to the respective examples, thecolumn of the side number Si in the lens data depicted in FIG. 3 andFIG. 5 shows the number of (i)th side (i=1 through 8) to which thesymbol is affixed with the side of the extreme object side componentmade into the first side so that the symbol is gradually increasedtoward the image side. The column of the curvature radius Ri shows avalue of the curvature radius of the (i)th side from the object side,corresponding to the symbol Ri affixed in FIG. 1 and FIG. 2. The columnof the between-surface distance Di shows the interval on the opticalaxis between the (i)th surface Si from the object side and the (i+1)thsurface side Si+1 as well. The unit of values of the curvature radius Riand the between-surface distance Di is millimeters (mm). The columns ofNdj and vdj show the values of the refractive index and Abbe numbercorresponding to the d-line (wavelength: 587.6 nm) of the (j)th (j=1through 4) optical element from the object surface including an opticalcomponent GC. FIG. 3 and FIG. 5 also show the values of paraxial focaldistance f (mm) of the entire system, F-number (FNO.), and angle of view2 ω (ω: half angle of view).

Further, in the single focus lens according to Example 1, the aperturestop St is disposed at the position of 0.15 mm from the object surfaceside of the first lens G1 to the plane side on the optical axis Z1. Inthe single focus lens according to Example 2, the aperture stop St isdisposed at a position of 0.05 mm from the object surface side of thefirst lens G1 to the plane side on the optical axis Z1.

In the respective lens data in FIG. 3 and FIG. 5, the symbol (*) affixedat the left side of the surface number shows that the lens surface ismade aspheric. In all single focus lenses according to the respectiveexamples, both sides of the first lens G1, the second lens G2 and thethird lens G3 are made aspheric. The basic lens data in FIG. 3 and FIG.5 show the numerical values of curvature radii in the vicinity of theparaxial axis of the optical axis as the curvature radii of theseaspheric surfaces.

FIG. 4 and FIG. 6 show the aspheric surface data in the single focuslenses according to Examples 1 and 2. For the numerical values depictedas the aspheric surface data, the symbol “E” shows that the successivenumerical value is a “power index” with 10 made into the lower numericalvalue. The numerical value that is displayed with the exponentialfunction designated as 10 as the lower numerical value is multipliedwith the numerical value in front of E is shown. For example, [1.0E-02]shows [1.0×10⁻²].

As the aspheric surface data, values of the respective coefficientsA_(i) and K in the expression of the aspheric surface shape, which isexpressed by the following expression (A) are shown. In further detail,Z shows the length (mm) of a perpendicular line from the point on theaspheric surface, which is located at a height h from the optical axis,down to the tangential plane (plane vertical to the optical axis) of thetop point of the aspheric surface. In the respective single focus lensesaccording to the respective examples, the respective aspheric surfacesare expressed by effectively utilizing even number order coefficientsA₄, A₆, A₈, A₁₀ and odd number order coefficients A₃, A₅, A₇, A₉.Z=C·h ²/{1+(1−K·C ² ·h ²)^(1/2) }+A ₃ ·h ³ +A ₄ ·h ⁴ +A ₅ ·h ⁵ +A ₆ ·h ⁶+A ₇ ·h ⁷ +A ₈ ·h ⁸ +A ₉ ·h ⁹ +A ₁₀ ·h ¹⁰  (A)where

-   -   Z is a depth (mm) of the aspheric surface,    -   h is a distance (height)(mm) from the optical axis to the lens        side,    -   K is eccentricity,    -   C is a paraxial curvature=1/R (R is a paraxial curvature        radius), and    -   A_(i) is the (i)th aspheric surface coefficient

FIG. 7 shows values in the respective conditional expressions describedabove, as aggregates in regard to the respective examples. As has beenunderstood in FIG. 7, the values of the respective examples are withinthe numerical ranges of the respective conditional expressions.

FIG. 8A through FIG. 8C show the spherical aberration, astigmatism, anddistortion in the single focus lens according to Example 1. In therespective aberration views, aberrations are depicted with the d-linemade into the reference wavelength. However, in the spherical aberrationviews, aberrations are shown in regard to the g-line (wavelength 435.8nm) and c-line (wavelength 656.3 nm). In the astigmatic views, the solidlines show aberrations in the sagittal direction, and the broken linesshow aberrations in the tangential direction. ω shows the half angle ofview. Similarly, various aberrations of the single focus lens accordingto Example 2 are shown in FIG. 9A through FIG. 9C.

As has been understood from the respective numerical data and variousaberration views described above, with the respective examples, it ispossible to achieve a compact lens system for which the aberrations aresatisfactorily corrected while being provided with a fewer number oflenses.

Also, the present invention is not limited to the above-describedembodiment and the respective examples. The present invention may besubjected to various modifications. For example, values of the curvatureradius, between-surface distance and refractive index of the respectivelens components are not limited to the values shown in the respectivenumerical examples, but may take other values.

With the single focus lens according to the present invention, a singlefocus lens is provided with a first lens of positive power having aconvex shape at the surface of the object side, a second lens of anegative meniscus lens having a concave shape at the surface of theobject side on the paraxial axis, and a third lens of an aspheric lenshaving a convex shape at the surface of the object side on the paraxialaxis, and the single focus lens satisfies the predetermined conditionalexpressions, wherein since vitreous materials of the respective lensesare attempted to be optimized while securing the compactness, it ispossible to achieve a compact lens system of high performance whilebeing provided with only three lenses.

The entire disclosure of each and every foreign patent application fromwhich the benefit of foreign priority has been claimed in the presentapplication is incorporated herein by reference, as if fully set forth.

1. A single focus lens comprising: a first lens of positive power havinga convex-shaped surface on an object side; a second lens of a negativemeniscus lens having, on the object side, a concave-shaped surface onits paraxial axis; and a third lens of an aspheric lens having, on theobject side, a convex-shaped surface on its paraxial axis, in this orderfrom the object side, wherein the single focus lens satisfies thefollowing conditions:1.5>f1/f>0.6  (1)24.0<vdA<32.0  (2)55.5<vdB<58.5  (3)1.55<NdA  (4)1.47<NdB<1.55  (5)1.8>L/D  (6)0.30<RA/f<0.40  (7) where f: Entire focal distance, f1: Focal distanceof the first lens, vdA: Abbe number of the second lens, vdB: Abbe numberof the first lens and the third lens, NdA: Refractive index at d-line ofthe second lens, NdB: Refractive index at d-line of the first lens andthe third lens, RA: Paraxial curvature radius of a surface at the objectside of the first lens, L: Distance on the optical axis from the surfaceat the object side of the first lens to an imaging position, and D:Maximum image height.
 2. The single focus lens according to claim 1,further comprising an aperture stop disposed at the object side from thesurface at an image side of the first lens on the optical axis.